The present invention relates to the communication of optical frequency modulated signals over an optical communication path and more particularly, to the reception and demodulation of amplitude modulated video signals from an optical fiber link. The invention has particular applicability to the reception of video signals in an optical fiber cable television system.
Recently, there has been a growing interest in the development of analog optical communication systems. In comparison with digital systems, analog communication systems provide an efficient use of bandwidth. This is particularly useful in cable television (CATV) transmission system applications, where it is necessary to transmit a large number of video channels through an optical fiber. Compatibility with existing equipment is achieved by using the same amplitude modulated vestigial-sideband (AM-VSB) signal format for optical transmission that is in use for coaxial cable signal transmission.
The use of a television signal comprising AM-VSB video subcarriers is preferred in the distribution of cable television signals due to the compatibility of that format with National Television System Committee (NTSC) television standards and the ability to provide an increased number of channels within a given bandwidth. An undesirable characteristic of AM-VSB transmission is that it requires a much higher carrier-to-noise ratio (CNR) than other techniques, such as frequency modulation or digital transmission of video signals. Generally, a CNR of at least 40 dB is necessary to provide clear reception of AM-VSB television signals.
In order to transmit an information signal (e.g., a television signal) over an optical fiber, a light beam ("carrier") must be modulated by the information signal. Certain types of lasers providing optical carriers can be directly modulated, as well known in the art. Alternatively, external modulation of a laser beam can be provided. Each of these approaches has well known advantages and disadvantages.
To date, commercial deployment of fiber optic analog multichannel communications systems has been dominated by directly modulated laser source technology. Although there are potential benefits with external modulation of a continuous wave (CW) laser source, external modulators such as Mach Zehnder waveguide modulators (interferometers) are highly nonlinear. This characteristic poses severe limitations on the dynamic range that can be achieved in analog communication systems. On the other hand, external modulator links can be designed which virtually eliminate frequency chirping and which can be used with high power shot noise limited solid state lasers. Thus, the development of optical communication systems using external modulators would be advantageous, if the disadvantages of prior art external modulation techniques can be overcome.
Most optical components are capable of reciprocal operation. A Mach Zehnder modulator, in particular, can be used both to modulate signals onto a carrier and to discriminate (i.e., demodulate) signals from a carrier, if the device is provided in an unbalanced form. It is known, for example, that an unbalanced Mach Zehnder modulator will discriminate optical frequency modulation (FM). However, to date it has been difficult to optically discriminate optical frequency modulation in this manner, due to the limited dynamic range over which the linearity of the Mach Zehnder modulator is sufficient for communication links that require very linear operation. In particular, multichannel video is an application that requires a very linear demodulator.
A Mach Zehnder modulator has an operating characteristic (intensity versus frequency) that follows a cosine.sup.2 curve. Typically, the Mach Zehnder modulator is biased to operate at quadrature, which is the point where the output intensity of the device is at 50% of its maximum value. This point is centered within an approximately linear region of the cosine.sup.2 curve.
Frequency modulation of AM signals over an optical communication path has several advantages. For example, FM communication has much lower CNR and distortion product requirements. Multiple channel video transmission has been demonstrated using individual subcarriers for each of a plurality of FM modulated video channels. Practical optical FM discriminators are necessary, however, in order to demodulate the AM information signals from the frequency modulated optical carrier. Although an unbalanced Mach Zehnder discriminator would be useful in demodulating such signals, it is necessary to improve the dynamic range of such a discriminator in order to provide a practical system.
It would be advantageous to provide a method for linearizing an unbalanced Mach Zehnder optical frequency discriminator. It would be further advantageous to provide such a method that effectively suppresses third order nonlinear distortion introduced by the discriminator. It would also be advantageous to provide such a method that avoids signal degradation due to second order nonlinearities. The present invention provides methods enjoying the aforementioned advantages.